Engines that transform the internal energy within a pressurized expansible fluid into useful mechanical energy are well known. Perhaps the earliest and best known is the steam engine. Central to the operation of such an engine is the valve mechanism that controls the admission of high-pressure fluid into an expansible chamber and the release of low-pressure fluid from the expansible chamber. The power and efficiency of such an engine is strongly driven by the phasing of the opening and closing of the inlet and outlet valves. Maintaining high efficiency and high power under a variety of pressure conditions and operating speeds requires changing the phasing of the valves opening and closing, and a number of mechanisms are known to achieve such variable valve timing. However, known mechanisms tend to be complex and expensive to manufacture, and there is a need for a simple valve mechanism that is inexpensive to manufacture and that has high reliability and is capable of changing operation in response to engine speed and pressure.